1. Field of the Invention
This invention relates to an automatic white balance adjusting device for an electronic still camera, a video camera, or the like.
2. Description of the Prior Art
Usually, white balance adjustment is performed in a video camera so as to correctly reproduce white objects as white. In the prior art, white balance adjustment is performed such that an average of data in a video signal per frame indicates an achromatic color. However, when most of an image is chromatic, a white object tends to be erroneously white-balanced in this method. This phenomenon is called "color failure". To overcome this problem, there have been proposed a number of white balance adjusting methods. One of them is disclosed in Japanese Patent Laid-Open Publication No. Hei 5-292,533. In this method, a video signal is divided into a plurality of blocks, representative values of predetermined areas in the blocks are white-balance adjusted such that an average of these representative values represents an achromatic color. Japanese Patent Laid-Open Publication No. Hei 5-007,369 proposes a method in which a limited number of white balancing signals are used so as to white-balance images in a limited range.
A white object which is present indoors under a fluorescent lamp tends to be reproduced as a greenish white color. Thus, it is difficult to distinguish such a greenish white object from green turf under daylight. Color failure tends to occur in such a case. The method of the second cited reference can prevent the color failure in this case. Specifically, it is checked, according to the brightness of the object, whether the object is present outdoors or indoors.
U.S. Pat. No. 4,736,241 discloses a white balance adjusting method, in which a white balance adjusting signal is weakened if a color temperature is very low in sunset when it is difficult for the human eyes to adapt.
However, the conventional white balance adjusting methods do not seem to have paid any attention to the following problems.
In Japanese Patent Laid-Open Publication No. Hei 5-007,369, if an object is in a dimly lit room without any artificial light source such as a fluorescent lamp, but bright sunlight is incident into the room via a window, the object is judged to be indoors. Then, the white balance adjustment is performed assuming that the object is indoors under a fluorescent or tungsten lamp. However, no white balance adjustment is conducted with respect to bright sunlight. Further, when two light sources such as a fluorescent lamp and sunlight are present, the object is simply considered to be present indoors. In such a case, the white balance adjustment is performed with respect to the fluorescent or tungsten lamp, which does not seem appropriate for the object.
If the white balance adjustment of U.S. Pat. No. 4,736,241 is applied to the Japanese Patent Laid-Open Publication No. Hei 5-292,533, when the object under dull sunlight is illuminated by the fluorescent lamp, the white balance adjustment is performed to remove the influence of the fluorescent lamp. This means that the white balance adjusting signals is weakened, and that the white balance adjustment is not precise.